Method for annealing magnetic wire

ABSTRACT

A conductive wire which is plated with a magnetic coating is annealed in an oxygen environment. The resulting oxides are removed to provide good electrical contact for online testing.

United States Patent 1151 3,637,443 Ransom 14 1 Jan. 25, 1972 [54] METHOD FOR ANNEALING 3,348,983 10/1967 Odani et a1 1.148/121 x MAGNETIC WIRE 3,279,959 10/1966 Oshima et al.. ..148/103 3,451,793 6/1969 Matsushita ..29/ 194 [72] Inventor: Lloyd D. Ransom, Torrance, Calif. 3,370,929 2/1968 Mathias .29/194 X [73] Assignee: Motorola Primary Examiner-Byland Bizot [22] Filed: Nov. 28, 1969 Assistant Examiner-G. K White 1 pp] No: 880,860 Attorney-V1ncent Rallllilial'ld Henry Olsen 0 m [52] U.S. C1 ..l48/l22, 29/194, 148/31.55,

148/ 1 21 [57] ABSTRACT gsi A conductive wire which is plated with a magnetic coating is 1 108 5, 3 annealed in an oxygen environment. The resulting oxides are removed to provide good electrical contact for online testing.

[56] References Cited UNITED STATES PATENTS l-layasaka et a1. 39/19432 1 Claims, 9 Drawing Figures METHOD FOR ANNEALING MAGNETIC WIRE This invention relates to plated wires for magnetic memories and more particularly to a method of annealing such wires.

According to present techniques, a nonmagnetic electrically conductive wire or substrate is plated with a magnetic coating. Such coating has a substantially rectangular hysteresis loop or magnetization curve. Normally, the wire is annealed in an inert atmosphere. Applicant has discovered that by intentionally annealing the plated wire in an oxidizing atmosphere the magnetic characteristics can be substantially improved.

Therefore, an object of the invention is to provide a plated wire having improved parameters.

Another object of the invention is to provide a simplified method for annealing wire.

A further object of the invention is to provide a plated wire having improved nondestructive readout (NDRO) characteristics.

These and other objects will be better understood by reference to the following detail description when taken with the drawings in which:

FIG. 1 is a curve showing the relationship between the drive current and output voltage;

FIGS. 2-9 are reproductions of actual oscillograms helpful in explaining the invention.

In the operation of plated wire, the wire element itself normally serves as a sense digit line. In a comprehensive element test three solenoid or word lines are placed around the digit line. Disturb effects are achieved by attempting to write ad verse information into the left and right adjacent cells using the two outside word straps. The stored information is written into the center word strap on the same wire. The primary disturb phenomena are known as creep, crawl, and unipolar disturb. Creep is that effect which is produced by writing adverse information into the adjacent cells. Any deterioration of the information originally stored in the cell under test is known as creep disturb. The crawl phenomenon is a test utilizing only the word strap in the cell under test. In this case, a normal word disturb field is used in conjunction with a small amount of adverse digit current. The amount of current which is used in the digit line is called the crawl current and is usually small. The ability of the wire to withstand crawl can be related to its ability to withstand long term lifetime conditions sometimes known as aging effects. The unipolar disturb is a function only of the wire element itself and is independent of the word straps. The unipolar disturb threshold is the amplitude at which a unipolar pulse down the wire will destroy any information which has been written into the element. A combination of these three disturb conditions in a usable region will determine the stability of a plated wire element. The higher disturb thresholds provide a wider operating range of the plated wire element.

A useful type of test for characterizing these disturb effects is called a window test. A digit window (FIG. 1) is obtained by writing a hard history into the cell (bit) under test and into each adjacent cell, then an attempt is made to write a single time with a fixed word current and a digit current which is gradually varied from ma. to 100 ma. The output after each single write is displayed on the vertical axis of an oscilloscope while the digit current is displayed as the horizontal sweep. A constant readout current is used. The solid curve of FIG. I is the envelope of the outputs. Region A of FIG. 1 shows the result of hard history and the inability of a single write to overcome history. In Region 13 the single write becomes large enough to write into the cell under test. The current which is just large enough to reverse the effect of history (point P) is called the digit pop point.

In order to study the effect of fringe fields from adjacent word straps, a disturb step is added between the single write and the read out. Along with the adjacent word currents, digit disturb is added so that information of opposite polarity to that in the cell under test is written into both adjacent cells. This digit disturb current is varied from 0 ma. to 100 ma. at

the same time as the digit write current. As this disturb current becomes large enough, the information in the cell under test is destroyed as is shown by the dashed curve in Region C.

FIG. 2 shows a reproduction of a typical window and shows both polarities. It can be seen that the digit pop point occurs at 22 ma., that an output ofil4 mv. is obtained at 50 ma. of digit drive. For these tests, a two-turn word strap consisting of two 0.010 inch wide straps separated by a 0.005 inch airgap was used. The word strap pairs were spaced 0.050 inch center to center. The substrate wire is 0.005 inch diameter Be-Cu having a luCu coating and a l Ni-Fe coating. The permalloy film is at the -20 zero magnetostriction point. The fixed word currents were 450 ma. for write and read and 550 ma. for history and disturb. A 48 ma. fixed digit current was used for history.

Another type of window which is very useful in studying plated wire is called the crawl window. In this case, all of the word and digit currents are held fixed while the crawl current is varied from 0 to 10 ma. and the point at which the crawl current becomes large enough to destroy the information stored can be seen. FIG. 3 is a typical crawl window showing the disturb point.

In the normal preparation of plated wire memory elements, the wire is annealed in an inert atmosphere at between 300 and 400 C. in order to stabilize its magnetic properties against aging. This operation results in very minor changes in the magnetic properties of the element. The digit windows showing creep and unipolar disturb efi'ects and the crawl window obtained when such wire is produced under standard conditions on a standard plating line are shown in FIGS. 4, 5 and 6, respectively. It can be seen that a crawl current less than I ma. can destroy the stored information and that the unipolar disturb threshold occurs at about 40 ma. It would be very desirable if these two values could be increased so that a more stable operating plated wire could be produced. However, utilizing standard technologies that is almost impossible to achieve.

Usually, the annealing step is performed in either an inert or reducing atmosphere in order to minimize oxidative effects to the wire. The essence of the present invention is that it has been discovered that a controlled oxidation of the plated wire during this annealing step is highly beneficial to the magnetic properties.

The curves in FIGS. 4, 5 and 6 were obtained from wire annealed in a pure nitrogen atmosphere with a 0 partial pressure of oxygen. FIGS. 7, 8 and 9, respectively, show the digit window with creep and unipolar disturbs, and the crawl window for wire prepared in an atmosphere containing approximately 20 percent partial pressure of oxygen. It can be seen that both the crawl and unipolar characteristics of the wire have been dramatically improved by this treatment. The utilization of a lower partial pressure of oxygen (i.e., 10 percent) obtained by mixing pure oxygen and an inert gas in the oven will produce a less dramatic result but a wire which is significantly superior to that prepared in an inert or reducing atmosphere. The utilization of larger than 20 percent partial pressures of oxygen will lead to successive improvements in both unipolar and crawl characteristics; however, ultimately the required digit current for utilization of the wire becomes too high to be practical. The optimum condition of oxygen in the oven depends upon the characteristics which are desired in the plated wire. However, for a nominal digit current in the usual range of 40 or 50 ma., a partial pressure of 20 percent of oxygen is approximately optimum. Other techniques could be utilized to form the oxide layer on the surface or within the plated wire; such as a periodic reverse current, the utilization of anodization techniques, or the utilization of oxidizing chemical materials.

The mechanism by which the oxidation of the plated wire memory elements improves the magnetic properties is not completely understood. However, the addition of oxygen to the annealing process results in a decrease of magnetic flux and a shift toward negative magnetostriction. These effects must result from oxidation of iron or iron and nickel to form metal oxides. The unipolar disturb threshold is also referred to as a wall motion coercive force. It is related to the easy axis field which is required to initiate wall motion switching in the easy axis of the plated wire element. Any material in the plated wire element which will retard motion of the walls or inhibit nucleation of new walls will increase the wall motion threshold. It is suggested that oxides are formed during the passage of the element through the annealing oven at 300 to 400 C. leaving a mixture of iron, nickel, and metal oxides on the surface of the wire. The wire is in the oven for approximately 1 minute. A second process which is a companion to the oxidation of the wire at the surface is a diffusion of the metal oxides into the interior of the wire. This diffusion, occurring at a temperature of 300 to 400 C., permits metal oxides to penetrate perhaps to the substrate material. As the metal oxides diffuse into the film, fresh material is exposed at the surface, thereby permitting the oxidation process to continue and to account for the surprising lack of saturation of the effect. Therefore, further oxygen continues to produce more oxide on a surface which should have been completely oxidized. The metal oxides now are distributed through the nickel-iron film and although these oxides are more concentrated near the surface they are diffused into the film-perhaps all the way through to the substrate. This oxide material causes domain wall pinning and accounts for the high-wall motion threshold which is observed in the device. This wallpinning inhibits the crawl phenomenon and explains all of the magnetic properties which have been observed.

Therefore, instead of a nickel-iron film as in the prior art, a nickel-iron film with a controlled amount of metal oxides dispersed throughout results from this invention and such oxide is responsible for the enhancement of the magnetic properties in accordance with the invention over those obtained in a pure permalloy film. The control of the oxide is obtained through control of the oxygen partial pressure in the oven or through the amount of oxidation which is permitted in the element prior to its introduction into the oven.

Further control is obtained through the temperature of the annealing processes, since this will control the rate of diffusion into the rest of the film. The new device, therefore, is no longer a magnetostriction nickel-iron plated wire that has been utilized by others but is a zero magnetostrive nickel-ironmetal oxide film in which the oxide is present in controlled amounts, thereby producing a controlled enhancement in the magnetic properties of the device.

The introduction of the oxide surface on the plated wire causes one problem in the processing of the wire which is not present if the wire is produced by more conventional techniques. The oxide surface renders the wire either poorly conductive or nonconductive at its surface. This has no effect upon the utilization of the device in memory stacks; however, as a subsequent step to the wire processing it is normally passed between mercury contacts for an on-line electrical test. This procedure requires that the surface of the wire be conductive and that the contact resistance be very small. Therefore, as a further part of the invention, it is necessary to develop techniques for the continuous removal of the oxide surface from the wire prior to the on-line testing.

This can be accomplished in a number of ways either by removal of the oxide by chemical means or by reduction of the oxide on the surface layer through the utilization of a second oven containing a reducing atmosphere. The method which has been used in actual operation of the process described utilizes a hydrochloric acid solution which removes thin oxide layers from the surface of the wire. This is followed by a rinse in an organic solvent such as methanol or acetone to remove the water and acid from the surface of the wire prior to its passing through the mercury contacts for testing. This added step of cleaning the wire is necessary only because of the testing procedures which are used on-line and has no effect upon the magnetic properties of the element which have already been formed and determined by the annealing operation in the oven.

lclaim:

l. A method of annealing plated wire consisting of a beryllium copper conductive substrate (preferably beryllium copper and) having a magnetic coating of 0 magnetostrictive nickel iron, comprising (oxidizing) forming an oxide coating on said wire by annealing said wire in an oxidizing atmosphere for approximately 1 minute at 300 to 400 C., and then removing the resulting oxide surface layer to provide a conductive surface. 

